1. Field of the Invention
The present invention relates to a cathode ray tube (CRT), and more particularly, to a CRT having an improved mounting structure of an electron gun in a neck portion of the CRT.
2. Description of the Related Art
FIGS. 1 and 2 show a conventional CRT using the principle of a triode. As shown, the CRT includes a bulb 10 having a screen 12 coated with a fluorescent layer 11 on its inner surface, a conductive layer 13 coated on the inner surface of portions other than a neck portion 14 of the bulb 10, an electron gun 20 mounted in the neck portion 14 of the bulb 10 and having at least one final accelerating electrode 21 and a focusing electrode 22, and a deflection yoke 30 mounted on a cone portion 15 of the bulb 10. A high voltage of 25.about.29 kV is applied to the conductive layer 13 through an anode port 16 installed in the bulb 10.
Referring to FIG. 2 showing the neck portion 14 of the bulb 10, the conductive layer 13 is connected to a spacer 24 of a shield cup 23 fixed on the final accelerating electrode 21 of the electron gun 20 so that a high voltage is applied to the final accelerating electrode 21.
In the CRT having the aforementioned configuration, electron beams emitted from the electron gun 20 are selectively deflected by the deflection yoke 30 to excite phosphors of the fluorescent layer 11, thereby forming an image. During this process, excess thermions removed from the electron gun 20 but not used in exciting the phosphors are emitted to the outside through the conductive layer 13 and the anode port 16.
However, since the conductive layer 13 is not formed on the inner surface of the neck portion 14 into which the electron gun 20 is mounted, leakage current flows along the inner circumferential surface of the neck portion 14 due to the high voltage applied to the conductive layer 13. This leakage current adversely affects an electronic lens formed between electrodes in the electron gun 20. Further, if foreign matter is adsorbed into the inner circumferential surface of the neck portion 14, an arc discharge may occur between the foreign matter and the electrodes due to the leakage current.
In a projector for implementing a color image by projecting a monochrome image formed at unit CRTs for red, blue and green colors, a high-brightness image is required. To this end, it is necessary to apply a high voltage to a screen coated with a conductive layer and a fluorescent layer.
However, in the conventional CRT, as described above, leakage current may flow along the inner circumferential surface of a neck portion to break insulation, thereby causing discharge on the inner surface of the neck portion and the electrodes of the electron guns. Also, since the distance between a final accelerating electrode to which a high voltage is applied and a focusing electrode is as narrow as 1.about.2 mm, leakage current may flow through a bead glass to cause discharge.
To overcome the above-described problem, conventionally, high-resistance layers having different resistance values are sequentially coated on the inner circumferential surface of the neck portion to be used as a final accelerating electrode. In this case, however, coating the high-resistance layers is a difficult task.